Temporary array data for studying seismic anisotropy of Variscan Massifs - the Bohemian Massif, Armorican Massif and French Massif Central

(1) Geophysical Institute, Academy of Science of the Czech Republic, Prague
(2) EOST - Institut de Physique du Globe Strasbourg, France

Abstract

In co-operation between the geophysical institutions in Prague and Strasbourg, an array of mobile digital three-component stations was installed in the Bohemian Massif (BM) in April 1998, aiming at recording teleseismic body waves for anisotropic and tomographic studies of the deep lithosphere. The array consisted of six broad-band (BB) and two short-period stations. The field measurement of the array has been running for 8 months. Three of the BB stations continue their operation. A data basis of the records is complemented by waveforms provided by permanent observatories within the region of the BM. In spite of relative seismic silence during the field measurement, several good-quality waveforms have been recorded and a joint analysis of P residual spheres and shear wave splitting performed. Data analyses show different orientations of anisotropic structures within the southern (Moldanubian) part of the BM, and the northern and western parts. In the western part of the BM the change relates to the Eger Graben and its continuation to the west.

Seismic anisotropy of the lower lithosphere has been evaluated also for the Armorican Massif (AM) and French Massif Central (MC) from records of regional arrays designed for teleseismic tomographic studies (Granet et al., 1995). The subcrustal lithosphere of varying thickness, thickened beneath the Variscan Massifs to about 140 km, on average, is characterized by a large-scale seismic anisotropy of P- and S-wave velocities which can be interpreted by a systematic preferred orientation of olivine grains resulting in a coherent fabric with dipping (a,c) foliation planes (Babuska and Plomerova, 1992). In general, lateral changes of the lower lithosphere anisotropy of the Variscan Massifs are related to prominent sutures cutting the whole lithosphere such as the Saxothuringian/Moldanubian contact (Plomerova et al., 1998) in BM, South Armorican Shear Zone in AM (Judenherc et al., 1999) or Sillon Houiller fault zone in MC (Granet et al., 1998), as well as to the Limagne Graben in the MC and the Eger Graben in the BM.

Whole poster

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References
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